What makes you think that they are filtering for good physicians? Or even filtering for those who are capable of becoming doctors?
Here in South Africa, post-1994, universities adopted a very aggressive affirmative action policy to correct imbalances of the past. While med-schools had previously been based purely on merit, post-1994 introduced quotas for each of the major race groups.
This meant that a disadvantaged-background student with a C average would often be picked over an advantaged student with an A average.
The result was still doctors who graduated, and then went on to practice. The restriction by med-schools everywhere is not because only high-performing students will survive it, it's a purely artificial restriction.
After all, if C average students pass, why not simply increase the intake?
Think of it this way; in Europe it is far easier to become a doctor then in the US due to significantly less stringent requirements. However in Europe and other 1st world countries treatment and other quantitative measures on quality of medical care beat the US by a far margin.
It shows that the "unpleasantness" is pushing something to the right of the bell curve. However it is clearly not pushing "effective medical diagnosis and treatment" to the right of the curve as Europe is beating us on that front with far less stringent requirements.
[1] It's supposed to be legally limited to 80, but ask around and it's obvious that even that limit isn't properly enforced.
The big problem is that 2 is true.
In fact, education and medical systems are different enough between Europe and the US that saying one is harder is very subjective. For instance, american specialists are really hyperspecialized. They often excel at what they do, and are completely ignorant of other fields. Europeans often have a much broader medical knowledge. They are also far less aggressive. All in all, I much prefer Europe.
Either way if you're saying the "hardness" is the same, the conclusion still stands. "Hardness" is not the metric that is making European quality of care better than the US because both countries have the same "hardness".